Cassini reveals the secret of Saturn's moon Mimas

A mosaic of Saturn's moon Mimas taken by Cassini during a flyby on 13 February 2010.

Credit:NASA/JPL-Caltech/Space Science Institute

Mimas, one of Saturn’s moons, has a secret hidden beneath its icy surface. Observations taken by NASA’s Cassini mission determined how much the moon wobbles as it orbits the planet, finding it moves double the amount predicted.

After modelling the satellite, a team of researchers came up with two possibilities of how the interior might be arranged. Either the moon’s frozen core is shaped like a rugby ball, or it contains an ocean of liquid water. Either possibility would be fascinating.

The wobble is caused by an effect called libration. The moon’s orbit is elliptical, meaning that to an observer on Saturn its speed across the sky would change very slightly over the course of an orbit. This means that even though the moon is gravitationally locked, so that the same face is always pointing towards the planet, small amounts of terrain along the limb come in and out of view over the month due to the difference in speed.

"Observing libration can provide useful insights about what is going on inside a body," said Radwan Tajeddine, a Cassini research associate from Cornell University. "In this case, it is telling us that this cratered little moon may be more complex than we thought."

If the interior is shaped like a rugby ball this could have interesting implications for how Mimas first formed. The moon is more than four billion years old, so the core should have relaxed into a more or less spherical shape by now. If it hasn’t, then an elongated shape could be left over from when the moon was created. However, simulations show that an oblong core would result in Mimas having a different shape to what is observed.

If instead Mimas possesses an ocean then it would join an exclusive club of worlds with liquid water. It would be a surprise to find a global ocean on the icy body, as the surface of Mimas does not display signs of geologic activity that you would expect to find on a world with a fluid layer. Future models and observations from Cassini should help answer what lies beneath the moon.

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